Everscream

It only exists inside the Frost Moon, which you start yourself with a Naughty Present after dark once Plantera is down. It holds off until the fourth wave and then keeps turning up, and it is worth a large chunk of wave points on death, so killing it is the fastest way to push the counter toward the waves that actually drop things.

It is the Frost Moon's version of Mourning Wood and behaves almost identically, so if you have run a Pumpkin Moon the shape of the fight is already familiar.

It alternates between two things. Pine needles come out in fast, sloppy volleys that arc and drop short, less accurate than the flaming wood the Mourning Wood throws but coming much quicker. Ornaments bounce along the ground and burst after a few seconds into shrapnel that hurts on its own, in the same way a Crystal Bullet does.

Neither one passes through blocks, and that single fact decides the whole fight. Put a ceiling between you and it, stand on platforms above the range of both, and there is very little left it can do to you. It will still float up through terrain to reach you, so leave yourself room to move rather than sealing yourself into a box.

Unlike its Pumpkin Moon counterpart it never starts hitting harder as it gets low, so the last quarter of the fight looks exactly like the first. That makes it one of the safer things to leave half dead while you handle a wave, which is not true of most event bosses.

Its loot chances improve in later waves, so an early one is a source of wave points rather than a payday and there is no reason to draw the fight out. The later ones are what you actually want, which is another argument for clearing the early waves fast.

When the moon ends it runs for it along with everything else, but it stays killable on the way out and barely fights back, which is not true of Mothron and the other event bosses that go invincible. Chasing one down in those last seconds is a free kill and a free roll on its drops.

The name is evergreen crossed with scream. Its needle volley is the enemy side of the Razorpine it drops, and it is one of the very few event bosses that will never hand you a ranged weapon.